Image sensor and method for manufacturing the same

ABSTRACT

Provided is an image sensor and a method for manufacturing the same. In the image sensor, a first substrate has a lower metal line and circuitry thereon. A crystalline semiconductor layer contacts the lower metal line and is bonded to the first substrate. A photodiode is provided in the crystalline semiconductor layer and electrically connected with the lower metal line. A pixel isolation layer is formed in regions of the photodiode.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit under 35 U.S.C. §119 ofKorean Patent Application No. 10-2007-0090978, filed Sep. 7, 2007, whichis hereby incorporated by reference in its entirety.

BACKGROUND

In general, an image sensor is a semiconductor device that converts anoptical image to an electric signal. Image sensors are generallyclassified as a charge coupled device (CCD) image sensor or acomplementary metal oxide silicon (CMOS) image sensor (CIS).

In a related art image sensor, a photodiode is formed in a substratewith transistor circuitry using ion implantation. As the size of aphotodiode reduces more and more for the purpose of increasing thenumber of pixels without an increase in chip size, the area of a lightreceiving portion reduces, so that an image quality reduces.

Also, since a stack height does not reduce as much as the reduction inthe area of the light receiving portion, the number of photons incidentto the light receiving portion also reduces due to diffraction of light,called airy disk.

In a horizontal type CMOS image sensor according to the related art, aphotodiode and a transistor are horizontally formed adjacent to eachother on a substrate. Therefore, an additional region for the photodiodeis required, which may decrease a fill factor region and limit thepossibility of resolution.

Also, the horizontal type CMOS image sensor according to the related artmay cause a crosstalk problem between pixels.

Moreover, in the horizontal type CMOS image sensor according to therelated art, it is very difficult to achieve the optimized process ofconcurrently forming the photodiode and the transistor.

As an alternative to overcome this limitation, an attempt of forming aphotodiode using amorphous silicon (Si), or forming a readout circuitryon a Si substrate and forming a photodiode on the readout circuitryusing a method such as wafer-to-wafer bonding has been made (referred toas a “three-dimensional (3D) image sensor). The photodiode is connectedwith the readout circuitry through a metal line.

BRIEF SUMMARY

Embodiments of the present invention provide an image sensor and amethod for manufacturing the same that can provide a verticalintegration of a transistor circuitry and a photodiode.

Embodiments also provide an image sensor and a method for manufacturingthe same that can employ a vertical type photodiode capable ofinhibiting crosstalk between photodiode pixels.

Embodiments also provide an image sensor and a method for manufacturingthe same that can improve resolution and sensitivity.

Embodiments also provide an image sensor and a method for manufacturingthe same that can employ a vertical type photodiode having reduceddefects in the vertical type photodiode.

In one embodiments an image sensor can comprise: a first substratehaving a lower metal line and a circuitry formed thereon; a crystallinesemiconductor layer contacting the lower metal line and bonded to thefirst substrate; a photodiode provided in the crystalline semiconductorlayer and electrically connected with the lower metal line; and a pixelisolation layer in the photodiode.

In an embodiment, a method for manufacturing an image sensor cancomprise: preparing a first substrate having a lower metal line and acircuitry thereon; preparing a second substrate having a photodiodethereon; forming a pixel isolation layer in the photodiode of the secondsubstrate; bonding the first substrate to the second substrate such thatthe photodiode in which the pixel isolation layer is formed electricallycontacts the lower metal line; and removing a lower portion of thebonded second substrate to leave the photodiode on the first substrate.

In another embodiment, a method for manufacturing an image sensor cancomprise: preparing a first substrate having a lower metal line and acircuitry thereon; preparing a second substrate having a photodiodethereon; bonding the first substrate to the second substrate such thatthe photodiode contacts the lower metal line; removing a lower portionof the bonded second substrate to expose the photodiode; and forming apixel isolation layer in the exposed photodiode.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features will be apparent fromthe description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an image sensor according to a firstembodiment.

FIGS. 2 to 8 are cross-sectional views illustrating a method formanufacturing an image sensor according to the first embodiment.

FIG. 9 is a cross-sectional view of an image sensor according to anotherembodiment of the first embodiment.

FIG. 10 is a cross-sectional view of an image sensor according to asecond embodiment.

FIGS. 11 to 15 are cross-sectional views illustrating a method formanufacturing an image sensor according to the second embodiment.

FIG. 16 is a cross-sectional view of an image sensor according toanother embodiment of the second embodiment.

FIG. 17 is a cross-sectional view of an image sensor according to athird embodiment.

FIG. 18 is a cross-sectional view illustrating a method formanufacturing an image sensor according to the third embodiment.

FIG. 19 is a cross-sectional view of an image sensor according toanother embodiment of the third embodiment.

DETAILED DESCRIPTION

Embodiments of an image sensor and a method for manufacturing the samewill be described in detail with reference to the accompanying drawings.

In the description of embodiments, it will be understood that when alayer (or film) is referred to as being ‘on’ another layer or substrate,it can be directly on another layer or substrate, or intervening layersmay also be present. Further, it will be understood that when a layer isreferred to as being ‘under’ another layer, it can be directly underanother layer, or one or more intervening layers may also be present. Inaddition, it will also be understood that when a layer is referred to asbeing ‘between’ two layers, it can be the only layer between the twolayers, or one or more intervening layers may also be present.

FIG. 1 is a cross-sectional view of an image sensor according to a firstembodiment.

An image sensor according to the first embodiment can include: a firstsubstrate 100 having a lower metal line 110 and circuitry (not shown)thereon; a crystalline semiconductor layer 210 a (see FIG. 3) contactingthe lower metal line 110 and bonded to the first substrate 100; aphotodiode 210 provided in the crystalline semiconductor layer 210 a andelectrically connected with the lower metal line 110; and a pixelisolation layer 222 in the photodiode 210 to separate the photodiode 210according to unit pixel.

The image sensor according to an embodiment can reduce defects in aphotodiode by employing a vertical type photodiode in which thephotodiode is positioned on the circuitry, and forming the photodiode inthe crystalline semiconductor layer.

Also, the image sensor according to an embodiment can inhibit crosstalkbetween pixels due to an incident light by employing a vertical typephotodiode and forming the pixel isolation layer 222 between pixels. Inan embodiment, the pixel isolation layer 222 can be a shallow trenchisolation (STI) layer, but embodiments are not limited thereto.

Also, while FIG. 1 exemplarily shows that an upper width of the pixelisolation layer 222 is narrower than a lower width of the pixelisolation layer 222, embodiments are not limited thereto.

According to an embodiment, the crystalline semiconductor layer can be asingle crystalline semiconductor layer, but embodiments are not limitedthereto. For example, the crystalline semiconductor layer can be apolycrystalline semiconductor layer.

Although the circuitry of the first substrate 100 is not shown, in thecase of a CIS, the circuitry is not limited to a 4 Tr CIS having fourtransistors, but may be applied to a 1 Tr CIS, 3 Tr CIS, 5 Tr CIS, 1.5Tr CIS (transistor-shared CIS), or the like.

Also, the lower metal line 110 formed on the first substrate 100 caninclude a lower metal (not shown) and a lower plug (not shown). Anuppermost portion of the lower metal line 110 may function as a lowerelectrode of the photodiode.

The photodiode 210 can include a first conductive type conduction layer214 in the crystalline semiconductor layer 210 a, and a secondconductive type conduction layer 216 on the first conductive typeconduction layer 214 in the crystalline semiconductor layer.

For example, the photodiode 210 can include a lightly doped N-typeconduction layer 214 formed in the crystalline semiconductor layer 210a, and a heavily doped P-type conduction layer 216 formed in thecrystalline semiconductor layer, but embodiments are not limitedthereto. For example, the first conductive type is not limited toN-type, and may be P-type.

Also, in another embodiment as shown in FIG. 9, the photodiode 210 canfurther include a heavily doped first conductive type conduction layer212 beneath the first conductive type conduction layer 214 in thecrystalline semiconductor layer. The heavily doped first conductive typeconduction layer 212 can be formed for an ohmic contact to the firstsubstrate 100.

For example, the photodiode 210 can further include a heavily dopedN-type conduction layer 212 formed in the crystalline semiconductorlayer.

In a further embodiment, a top metal 240 can be formed on the photodiode210 and/or a color filter (not shown) can be formed on the photodiode210.

A method for manufacturing an image sensor according to the firstembodiment will now be described with respect to FIGS. 2 to 8.

As shown in FIG. 2, a first substrate 100 having a lower metal line 110and a circuitry (not shown) can be prepared. Although the circuitry ofthe first substrate 100 is not shown, in the case of a CIS, thecircuitry is not limited only to a 4 Tr CIS having four transistors asdescribed.

Also, the lower metal line 110 can include a lower metal (not shown) anda lower plug (not shown).

As shown in FIG. 3, a crystalline semiconductor layer 210 a can beformed on a second substrate 200. By forming a photodiode in thecrystalline semiconductor layer 210 a, it is possible to reduce defectsin the photodiode. In an embodiment, the second substrate 200 caninclude an insulating layer interposed between the second substrate 200and the crystalline semiconductor layer 210 a. In one embodiment, thecrystalline semiconductor layer 210 a can be epitaxially grown on thesecond substrate 200.

Alternatively, it is also possible to use an upper portion of the secondsubstrate 200 itself as a crystalline semiconductor layer in which aphotodiode is formed.

Next, as shown in FIG. 4, ions can be implanted into the crystallinesemiconductor layer 210 a to form a photodiode 210. For example, asecond conductive type conduction layer 216 can be formed at a lowerportion of the crystalline semiconductor layer 210 a. For example, aheavily doped P-type conduction layer 216 can be formed by performing ablanket ion implantation into the second substrate 200 (including thecrystalline semiconductor layer 210 a) into a lower portion of thecrystalline semiconductor layer 210 a without a mask. For example, thesecond conductive type conduction layer 216 may be formed having ajunction depth less than or equal to approximately 0.5 μm. The secondconductive type conduction layer 216 can be formed in the crystallinesemiconductor layer 210 a near a surface of the second substrate 200.

Thereafter, a first conductive type conduction layer 214 can be formedon the second conductive type conduction layer 216. For example, alightly doped N-type conduction layer 214 can be formed on the secondconductive conduction layer 216 by performing a blanket ion implantationinto an entire surface of the second substrate 200 without a mask. Forexample, the lightly doped first conductive type conduction layer 214can be formed having a junction depth ranged from approximately 1.0 μmto 2.0 μm.

In a further embodiment as shown in FIG. 9 a heavily doped firstconductive type conduction layer 212 can be formed on the firstconductive type conduction layer 214. For example, a heavily dopedN-type conduction layer 212 can be formed on the first conductive typeconduction layer 214 by performing a blanket ion implantation into anentire surface of the second substrate 200 without a mask. For example,the heavily doped first conductive type conduction layer 212 can beformed having a junction depth ranging from approximately 0.05 μm to 0.2μm.

Next, as shown in FIG. 5, a pixel isolation layer that can inhibitcrosstalk between pixels is formed in the crystalline semiconductorlayer 210 a in which the photodiode 210 is formed.

For example, the pixel isolation layer 222 can be formed by forming atrench T in the crystalline semiconductor layer 210 a in which thephotodiode 210 is formed, and filling the trench T with an insulatinglayer. For example, the pixel isolation layer 222 can be formed byforming a trench T in the crystalline semiconductor layer 210 a, forminga liner oxide layer (not shown) on the trench T, depositing an oxidematerial layer on the trench T and planarizing the resultant structure.

Next, referring to FIG. 6, the second substrate 200 can be bonded to thefirst substrate 100 such that the photodiode 210 of the second substrate200 having the pixel isolation layer 222 contacts the lower metal line110 of the first substrate 100.

For example, in one embodiment, the first substrate 100 and the secondsubstrate 200 can be bonded by contacting the first substrate 100 andthe second substrate 200 and then performing a plasma activation.However, embodiments are not limited thereto.

At this time, the bonding may be performed more easily when theinsulating layer 221 of the second substrate 200 and the interlayerinsulating layer of the first substrate 100 are made of the samematerial.

Also, when the first substrate 100 and the second substrate 200 arebonded to each other, the pixel isolation layer 222 and the lower metalline 110 are aligned such that the pixel isolation layer 222 does notcontact the lower metal line 110.

Next, as shown in FIG. 7, a portion of the bonded second substrate 200is removed to expose and leave the photodiode 210. For example, in thecase where the second substrate 200 includes an insulating layerinterposed between the second substrate 200 and the crystallinesemiconductor layer, the portion of the second substrate 200 can beremoved by back grinding, and the insulating layer exposed after theremoval of the lower portion of the second substrate 200 can be removedby an etch to leave only the photodiode 210 on the first substrate 100.

Alternatively, in the case where part of the second substrate 200 itselfis used as a crystalline semiconductor layer in which a photodiode isbeing formed, hydrogen ions (H⁺) can be implanted, before bonding thefirst substrate 100 to the second substrate 200, into the lower portionof the crystalline semiconductor layer in which a photodiode is formed,and the second substrate 200 can then be thermally annealed after beingbonded to the first substrate 100 to transform the hydrogen ion (H+)into hydrogen gas (H₂), thereby leaving only the photodiode and removingthe second substrate.

Next, as shown in FIG. 8, a top metal 230 can be formed on thephotodiode 210 and then a passivation (not shown) can be performed.Also, a color filter (not shown) can be further formed on the photodiode210, and a microlens can be further formed on the color filter.

Also, in a further embodiment, a transparent conduction layer (notshown) can be formed between the photodiode 210 and the top metal 240such that the top metal does not cover each pixel. In such embodiments,the transparent conduction layer performs the function of the top metal.In an embodiment, the transparent conduction layer can be formed ofindium-tin-oxide (ITO), but embodiments are not limited thereto. Thetransparent conduction layer may be formed to overlap at least two pixelregions. In other embodiments, the transparent conduction layer can beomitted.

The image sensor and the method for manufacturing the image sensoraccording to embodiments can reduce defects in a photodiode by employinga vertical type photodiode in which the photodiode is positioned on thecircuitry, and forming the photodiode in a crystalline semiconductorlayer.

Also, according to embodiments of the present invention, crosstalkbetween pixels can be inhibited by employing a vertical type photodiodeand forming a pixel isolation layer between the pixels.

FIG. 10 is a cross-sectional view of an image sensor according to asecond embodiment, and FIGS. 11 to 15 are cross-sectional viewsillustrating a method for manufacturing an image sensor according to thesecond embodiment.

The second embodiment can employ the technical characteristics of thefirst embodiment.

For example, the image sensor according to the second embodiment caninclude: a first substrate 100 having a lower metal line 110 and acircuitry (not shown); a crystalline semiconductor layer 210 a (see FIG.3) contacting the lower metal line 110 and bonded to the first substrate100; a photodiode 210 provided in the crystalline semiconductor layer210 a and electrically connected with the lower metal line 110; and apixel isolation layer 224 in the photodiode 210 separating thephotodiode 210 according to unit pixel.

The subject image sensor can reduce defects in a photodiode by employinga vertical type photodiode in which the photodiode is positioned on thecircuitry, and forming the photodiode in the crystalline semiconductorlayer.

Also, the subject image sensor can inhibit crosstalk between pixels byemploying a vertical type photodiode and forming the pixel isolationlayer 224 between the pixels.

Meanwhile, unlike in the first embodiment, in the second embodiment, thepixel isolation layer 224 is formed in the photodiode 210 after thefirst substrate 100 and the second substrate 200 are bonded.Accordingly, the pixel isolation layer 224 in the second embodiment canbe shaped such that its upper portion is wider than its lower portion.However, embodiments are not limited thereto.

In a specific embodiment, as shown in FIG. 11A, the first substrate 100having a lower metal line 110 and a circuitry (not shown) can beprepared. The lower metal line 110 can include a lower metal (not shown)and a lower plug (not shown).

In addition, as shown in FIG. 11B, a crystalline semiconductor layer 210a (see FIG. 3) can be formed on the second substrate 200. The photodiode210 can be formed in the crystalline semiconductor layer 210 a.

In one embodiment, the second substrate 200 can include an insulatinglayer interposed between the second substrate 200 and the crystallinesemiconductor layer 210 a.

In another embodiment, it is also possible to use an upper portion ofthe second substrate 200 itself as a crystalline semiconductor layer inwhich the photodiode is being formed.

The photodiode 210 can be formed by implanting ions into the crystallinesemiconductor layer 210 a.

For example, a second conductive type conduction layer 216 can be formedat a lower portion of the crystalline semiconductor layer 210 a.Thereafter, a first conductive type conduction layer 214 can be formedon the second conductive type conduction layer 216.

In a further embodiment as shown in FIG. 16 a heavily doped firstconductive type conduction layer 212 can be formed on the firstconductive type conduction layer 214.

Next, as shown in FIG. 12, the second substrate 200 can be bonded to thefirst substrate 100 such that the photodiode 210 of the second substrate200 contacts the lower metal line 110 of the first substrate 100.

For example, the first substrate 100 and the second substrate 200 can bebonded by contacting the first substrate 100 and the second substrate200 and then performing a plasma activation. However, embodiments arenot limited thereto.

Next, as shown in FIG. 13, a portion of the bonded second substrate 200is removed to expose and leave the photodiode 210 on the first substrate100.

For example, in the case where the second substrate 200 includes aninsulating layer interposed between the second substrate 200 and thecrystalline semiconductor layer, the portion of the second substrate 200can be removed by back grinding, and the insulating layer exposed afterthe removal of the portion of the second substrate 200 can be removed byan etch to leave only the photodiode 210 on the first substrate 100.

Alternatively, in the case where part of the second substrate 200 itselfis used as a crystalline semiconductor layer in which a photodiode isformed, hydrogen ions (H⁺) can be implanted, before bonding the firstsubstrate 100 to the second substrate 200, into the lower portion of thecrystalline semiconductor layer in which a photodiode is formed, and thesecond substrate 200 can then be thermally annealed after being bondedto the first substrate 100 to transform the hydrogen ions (H+) intohydrogen gas (H₂), thereby leaving only the photodiode and removing thesecond substrate.

Next, as shown in FIG. 14, a pixel isolation layer 224 that can inhibitcrosstalk between pixels can be formed in the photodiode 210 remainingon the first substrate 100 after removing the portion of the secondsubstrate 200.

For example, the pixel isolation layer 224 can be formed by forming atrench in the photodiode 210 and filling the trench with an insulatinglayer.

Next, as shown in FIG. 15, a top metal 240 can be formed on thephotodiode 210, and then a passivation may be performed. Also, a colorfilter (not shown) can be further formed on the photodiode 210, and amicrolens can be further formed on the color filter.

Also, in the present embodiment, a transparent conduction layer (notshown) can be formed between the photodiode 210 and the top metal 240such that the top metal is not formed in each pixel and the transparentconduction layer performs the function of the top metal. In anembodiment, the transparent conduction layer can be formed ofindium-tin-oxide (ITO), but embodiments are not limited thereto. In oneembodiment, the transparent conduction layer can be formed to overlap atleast two pixel regions.

FIG. 17 is a cross-sectional view of an image sensor according to athird embodiment.

The image sensor according to the third embodiment can employ thetechnical characteristics of the image sensor and the method formanufacturing the same according to the first embodiment.

Unlike in the first embodiment, in the third embodiment, the imagesensor can further include a second conductive type ion implantationlayer 221 between the photodiode 210 and the pixel isolation layer 222.

For example, the image sensor according to the third embodiment canfurther inhibit crosstalk between pixels of electrons or holes generatedin the photodiode 210 by interposing the second conductive type ionimplantation layer 221 between the photodiode 210 and the pixelisolation layer 222.

For this purpose, as shown in FIG. 18, the photodiode 210 can be formedin a single crystal layer of the second substrate 200, a trench T forthe pixel isolation layer can be formed, and then P-type ions can beheavily implanted to form a P+ ion implantation layer 221. Thereafter,the trench can be filled to form the pixel isolation layer 222.

Alternatively, after the pixel isolation layer 222 is formed, P-typeions can be heavily implanted into an interface between the photodiode210 and the pixel isolation layer 222 to form a P+ ion implantationlayer 221.

FIG. 19 is a cross-sectional view of an image sensor according toanother embodiment of the third embodiment.

In this embodiment, a heavily doped first conductive type conductionlayer 212 is further formed for an ohmic contact when the photodiode 210is formed.

According to the third embodiment, a second conductive type ionimplantation layer 221 can be formed together with the pixel isolationlayer 222, thereby more effectively inhibiting crosstalk between pixelsfrom electrons or holes generated in the photodiode 210.

Any reference in this specification to “one embodiment,” “anembodiment,” “example embodiment,” etc., means that a particularfeature, structure, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention. Theappearances of such phrases in various places in the specification arenot necessarily all referring to the same embodiment. Further, when aparticular feature, structure, or characteristic is described inconnection with any embodiment, it is submitted that it is within thepurview of one skilled in the art to effect such feature, structure, orcharacteristic in connection with other ones of the embodiments.

Although embodiments have been described with reference to a number ofillustrative embodiments thereof, it should be understood that numerousother modifications and embodiments can be devised by those skilled inthe art that will fall within the spirit and scope of the principles ofthis disclosure. More particularly, various variations and modificationsare possible in the component parts and/or arrangements of the subjectcombination arrangement within the scope of the disclosure, the drawingsand the appended claims. In addition to variations and modifications inthe component parts and/or arrangements, alternative uses will also beapparent to those skilled in the art.

1. A method for manufacturing an image sensor, comprising: preparing afirst substrate having a lower metal line and circuitry thereon;preparing a second substrate having a photodiode thereon; forming apixel isolation layer in the photodiode of the second substrate, thepixel isolation layer separating the photodiode according to pixels;bonding the first substrate to the second substrate such that thephotodiode in which the pixel isolation layer is formed electricallycontacts the lower metal line; removing a portion of the bonded secondsubstrate to leave the photodiode on the first substrate; and forming asecond conductive type ion implantation layer at lateral regions betweenthe photodiode and the pixel isolation layer, wherein the photodiode isformed vertically above the circuitry, wherein forming the pixelisolation layer comprises: forming a trench in the photodiodecorresponding to a region between pixels; and filling the trench with aninsulating layer, wherein forming the second conductive type ionimplantation layer comprises: implanting second conductive type ionsinto the photodiode at the region between pixels after tilling thetrench with the insulating layer to provide the second conductive typeions at the lateral region between the trench and the photodiode.
 2. Themethod of claim 1, wherein the preparing of the second substratecomprises: forming a crystalline semiconductor layer on the secondsubstrate; and forming a photodiode in the crystalline semiconductorlayer.
 3. The method of claim 2, wherein the forming of the photodiodein the crystalline semiconductor layer comprises: forming a secondconductive type conduction layer in the crystalline semiconductor layer;and forming a first conductive type conduction layer in the crystallinesemiconductor layer on the second conductive type conduction layer. 4.The method of claim 3, wherein the forming of the photodiode in thecrystalline semiconductor layer further comprises: after the forming ofthe first conductive type conduction layer, forming a heavily dopedfirst conductive type conduction layer in the crystalline semiconductorlayer on the first conductive type conduction layer.
 5. A method formanufacturing an image sensor, comprising: preparing a first substratehaving a lower metal line and a circuitry thereon; preparing a secondsubstrate having a photodiode thereon; bonding the first substrate tothe second substrate such that the photodiode contacts the lower metalline; removing a portion of the bonded second substrate to expose thephotodiode on the first substrate; forming a pixel isolation layer inportions of the exposed photodiode to separate the photodiode accordingto pixels; and forming a second conductive type ion implantation layerat lateral regions between the photodiode and the pixel isolation layer,wherein the photodiode is formed vertically above the circuitry, whereinforming the pixel isolation layer comprises: forming a trench in thephotodiode corresponding to a region between pixels; and filling thetrench with an insulating layer, wherein forming the second conductivetype ion implantation layer comprises: implanting second type ions intothe photodiode at the region between pixels after filling the trenchwith the insulating layer to provide the second conductive type ions ata lateral region between the trench and the photodiode.
 6. The method ofclaim 5, wherein the preparing of the second substrate comprises:forming a crystalline semiconductor layer on the second substrate; andforming a photodiode in the crystalline semiconductor layer.
 7. Themethod of claim 6, wherein the forming of the photodiode in thecrystalline semiconductor layer comprises: forming a second conductivetype conduction layer in the crystalline semiconductor layer; andforming a first conductive type conduction layer in the crystallinesemiconductor layer on the second conductive type conduction layer. 8.The method of claim 7, wherein the forming of the photodiode in thecrystalline semiconductor layer further comprises: after the forming ofthe first conductive type conduction layer, forming a heavily dopedfirst conductive type conduction layer in the crystalline semiconductorlayer on the first conductive type conduction layer.